AppVeyor
updated
to pre-release Python 3.8 in their default Ubuntu1804 image.
Since Python library wheels are often not available for beta Python versions, this can lead to CI test nuisance errors at setup.
Many projects will experience AppVeyor
errors on Numpy setup
as AppVeyor attempts to compile Numpy since the wheel doesn’t exist yet for pre-release Python versions.

Select Python version

AppVeyor has several Python versions installed.
The AppVeyor Python version selection method is OS-dependent.

The examples are shown specific to Windows and Linux for didactic clarity.
They would normally be merged to a single .appveyor.yml.